128 



J.A. ALLEN AND H.L. SANDERS 



Fig. 43. Propeleda louiseae. a & b, external lateral views of the right sides of shells of differing size to show differences in shell proportions with growth; 

 note outline internal morphology through semi-transparent shells, in particular the form of the hind gut and position of the adductor muscles; c, outline of 

 shell from the left side showing the outline of hind gut and adductor muscles; d, internal view of a left valve. All specimens taken from Atlantis II station 

 242 from the Argentine Basin. Scales = 1mm. 



Type specimen examined by HLS. 



Distributed at abyssal depths in the Argentine Basin; depth range: 

 3815-5223 metres. 



Shell description (Fig.43) 



Shell extremely elongate, slender, fragile, semi-transparent, 

 ornamented with moderately spaced concentric ridges, two post- 

 umbonal rounded ridges, one dorsal and one ventral at posterior 

 shell margin and crossed vertically by wavy continuations of the 

 concentric ridges, faint anterior radial ridge from umbo to antero- 

 ventral margin; umbo slightly raised, far anterior (post-umbonal 

 length 65-70% of total length), inwardly facing; antero-dorsal 

 margin with short proximal notch, distally margin raised and slightly 

 concave, faint angulation before anterior margin, anterior margin 

 joins with ventral margin in smooth curve, postero-ventral margin 

 very slightly sinuate, posterior margin angled and sinuate, postero- 

 dorsal margin notched at umbo, distally somewhat raised and keeled, 

 concave overall but slightly sinuous dorsal to hinge plate; hinge 

 plate relatively broad, posterior plate short occupying approxi- 

 mately half the postero-dorsal shell margin, hinge teeth elongate, 

 acute chevron shape, up to 12 in anterior and 20 in posterior series; 

 ligament small, internal, oblique, pear-shaped; rounded internal 

 ridge extends from umbo to posterior margin and marks junction 

 between inhalent and exhalent siphons. The maximum length of the 

 present specimens is 20. 3mm. The prodissoconch is large measuring 

 300um in length. 



Clarke (1961) records a long, thin, external ligament in the type 



specimen - the latter being a single large valve. We find no evidence 

 of an external part to the ligament and believe that Clarke mistakenly 

 confused periostracum along the postero-dorsal margin for liga- 

 ment. 



Internal morphology 



With one notable exception the internal morphology of Plouiseae 

 differs little from that of P. carpenteri. 



Unlike the latter species the hind gut of P. louiseae first passes to 

 the left side of the body where it forms a relatively small loop 

 immediately ventral to the umbo (Fig.43). From there it passes to the 

 right of the body and forms a loop that is considerably larger than 

 that on the left although not as extensive as that in P. carperteri (Figs 

 42 & 43). 



The adductor muscles are relatively large, the elongate poste- 

 rior muscle is situated at the distal limit of the posterior hinge 

 plate. The gill is very short and slender with few (c. 13) gill- 

 plates. 



Propeleda paucistriata (new species) 



Type specimen. Holotype BMNH 1995059; Paratypes BMNH 

 1995060. 



Type locality. Atlantis II station 203, Angola Basin, 08°48.00'S 

 12°52.00'E, 527-542m. 



